The invention relates generally to synchronous networks and, more particularly, to synchronization messaging schemes used in the management of synchronous networks.
Synchronization status messaging is used to manage the distribution of timing in synchronous networks. In particular, synchronization status messages (SSMs) are used to convey clock quality information so that network elements within a synchronous network can select the most suitable synchronization reference available in the network. Importantly, network elements rely on SSMs to trigger synchronization reconfigurations during fault conditions and restorations and to prevent the creation of timing loops during these reconfigurations.
Synchronization status messages have been defined for both Synchronous Optical Network (SONET) signals as well as Digital Signal Level 1 (DS1) Extended Superframe (ESF) signals. For example, see American National Standards Institute (ANSI) T1X1.3 Technical Report No. 33 (TR33), xe2x80x9cA Technical Report on Synchronization Network Management Using Synchronization Status Messagesxe2x80x9d, April 1994 and Bellcore Generic Requirements (GR-253-CORE), xe2x80x9cSynchronous Optical Network (SONET) Transport Systems: Common Generic Criteriaxe2x80x9d, pages 5-48 to 5-91, Issue 2, December 1995, Revision 1, December 1997, both of which are herein incorporated by reference. In general, SSMs are bit-oriented messages sent in a repeating fashion via 4 bits in the S1 overhead byte in SONET signals and via code words in the ESF data link for DS1 signals. According to the aforementioned standards, a valid ESF SSM is declared if 7 out of 10 xe2x80x9clikexe2x80x9d messages are received, e.g., 7 code words indicating the same quality level for a synchronization reference.
Typically, network elements in a synchronous network have several sources that can be used as synchronization references, e.g., external timing, terminating line signals, or internal clock. The preferable mode in most synchronous networks is the Building Integrated Timing Supply (BITS) concept for synchronization distribution whereby synchronized network elements within the network receive timing externally from a master timing supply, referred to as a BITS clock. However, synchronization reconfigurations caused by fault conditions and restorations may require a network element to select another synchronization reference. At present, SSMs are only used to convey quality levels of synchronization references to facilitate these reconfigurations.
It has become apparent that the existing SSM scheme defined in the aforementioned standards has not kept pace with the evolving complexity and size of today""s networks. Using SONET ring networks as an example, effective synchronization reconfigurations must take other factors into account besides the quality level of a synchronization references. For example, the direction of the timing supplied to a network element (e.g., east or west input signal being used to derive timing) or the number of available SONET timing sources may be important for the selection of a synchronization reference and for effectively preventing timing loops in the ring. This type of information is not transmitted by a network element to a BITS in the existing SSM scheme. Likewise, the identification of which network element""s derived DS1 is being used by the BITS as the timing reference may be important for synchronization management. Again, this type of information is not transmitted by a BITS to a network element in the existing SSM scheme.
In sum, the existing SSM scheme is inadequate to provide the amount and type of information that is needed to effectively manage synchronization in today""s synchronous networks.
An enhanced synchronization status messaging capability is provided according to the principles of the invention by supplying additional synchronization management information beyond that provided in existing synchronization messaging schemes but in a messaging format that is compatible with existing synchronization messaging standards. A plurality of additional messages, which are based on the same predefined code words used in existing synchronization status messages, are differentially coded and carried in available, but unused message positions in the existing messaging scheme.
In one illustrative embodiment, a two part message format is used for carrying information between network elements and between a network element and a co-located BITS clock in a synchronous network. A first part of the message format, referred herein as the standard message portion, carries the traditional quality level information of the synchronization references using the set of code words predefined in the applicable published standards. For example, the quality information of a synchronization reference is conveyed using 7 out of 10 xe2x80x9clikexe2x80x9d messages selected from the group of predefined code words set forth in the aforementioned standards. The second part of the message format, referred herein as the enhanced message portion, carries one or more enhanced messages that are conveyed using the same group of predefined code words as the standard message portion, but which are differentially coded into a plurality of message positions that are available and unused in the existing standard-compliant messaging schemes. For example, additional information is conveyed in the remaining 3 out of the 10 consecutive messages, which are presently transparent in the existing scheme. By differentially coding the enhanced message portion relative to the standard message portion, the enhanced message portion will always have separately identifiable code words even if the standard message portion changes, e.g., if the quality level designation of a reference conveyed by the 7 out of 10 xe2x80x9clikexe2x80x9d messages changes during a reconfiguration.
In addition to specifying the existing quality level designations for synchronization references using the standard message set, the enhanced synchronization status messages can be used to convey additional information, including, but not limited to: new quality level designations; reference, port and source designations; and node identification. The enhanced messaging scheme can also be used to carry out synchronization functions such as messaging for two-way time transfer and remote control to name a few.